This invention relates to inspection apparatus and methods for thin film measurement and more particularly to wafer inspection using either one or a combination of bright field, dark field or phase detection techniques.
Manufacturing of semiconductor devices involves several process steps including pattern imaging, ion implantation, pattern etching, film deposition, and chemical mechanical planarization. To provide high quality manufacturing, it is important to monitor process-tool performance and excursions in these processes. Therefore, an emerging need in the semiconductor industry is the need to inspect wafers-in-production (WIP) for yield limiting defects after each process step. The prior art/background description for this invention is, therefore, presented with reference to macro size surface defects on a patterned or an unpatterned semiconductor wafer specimen.
The objective of macro inspection is to ensure that the wafer is free from yield limiting large scale defects such as flakes, arc damaged areas, incomplete or extra photo-resist coverage, defects such as wagon wheel, comets, and striations in photoresist, defocus or hot spots, scratches, particles, residue, and nonuniform or incomplete edge bead removal.
One way to perform macro inspection is to manually observe the wafer under bright light while it is being rotated and nutated. U.S. Pat. No. 5,096,291 describes such an inspection method. Wafers are either passed or rejected based on operators"" judgement. Such manual inspection is very subjective, time consuming, and can be implemented only on sample basis. A better approach is to automate the macro inspection process as described in U.S. Pat. Nos. 5,917,588 and 5,777,729. While both patents describe use of bright field and dark field techniques for inspection, the latter uses diffracted light for its bright field inspection. This means that it is effective only in inspecting wafers with repetitive patterns, e.g., DRAM wafers. In both approaches, difference image computed from die to die subtraction or from golden wafer comparison, is used for automatic detection of macro defects. The detection sensitivity, however, is significantly influenced by noise in the difference image originating from die to die or wafer to wafer misalignment, die to die or wafer to wafer thickness variation, and under layer pattern noise.
The increasing monetary value of each wafer at every technology node makes any inspection strategy viable since, in many instances, the wafers can be re-worked and abnormal tool and process excursions detected and corrected in time. In addition, transition to 300-mm wafers, which can hold 2.25xc3x97 more chips, necessitates the need for continuous tool monitoring (via macro inspection of several wafers in every lot) as a way to minimize revenue loss. To achieve this level of inspection in manufacturing automated macro inspection tools with high throughput, better detection sensitivity, and repeatability are needed. In addition, for near real time feed back on the health of the process tools, these automated macro inspectors will have to co-locate within the process tools with other integrated metrology tools. Consequently, the inspector has to have smaller footprint compared to its stand-alone counterpart. To achieve higher throughput in a smaller footprint, the wafer is scanned using a combination of linear and rotary stage motion. Example of that is given in U.S. Pat. Nos. 6,320,609 and 6,407,809. The spiral scan method of image acquisition employed in the former may affect the homogeneity of the acquired image that could lead to false positive and reduced capture rate. The line scan method described in the latter limits detection sensitivity to 40xcexc+ defects. As like other automated defect inspectors, these two approaches also involve the process of sub-pixel registering of wafer to wafer images or adjacent die image fields and the computation of a difference image. Here again, the difference image is affected by noise originating from die to die or wafer to wafer misalignment, die to die or wafer to wafer thickness variation, and under-layer-pattern noise.
The present invention describes an inspection tool based on spiral-scan technique that is capable of generating images of product wafer surface independent of change in pattern orientation. In a product wafer, one comes across Manhattan geometry with L/S patterns that are usually smaller than the wavelength of illumination light used in most inspection tools. The wafer surface reflectance depends on incident polarization and the orientation of the plane of incidence with respect to the pattern. Consequently as the wafer rotates, the s-polarized light with its e-vector 5 as shown can go from being parallel to the pattern (classical orientation) to being perpendicular to the pattern (conical orientation). Usually s-polarized light is used instead of p-polarized as the former has higher reflectance than the latter at angles other than normal and grazing incidence. This leads to pattern dependent reflection that generates a bow tie-like surface reflectance as shown in FIG. 1. The bow tie feature degrades S/N ratio of the acquired image; thereby reducing defect detection sensitivity of the wafer inspector. One way to overcome this problem is to have another beam incident at the same spot with its incidence-plane orthogonal to the first one. This will create another bow tie pattern, but orientated 90 decrees (90xc2x0) with respect to the first one. If a detector senses both bow tie patterns simultaneously, the resultant image would be more uniform across the wafer surface as shown in FIG. 2.
The presently described invention provides for a method to illuminate substrate surface in two orthogonal incidence planes simultaneously or in tandem. This is done so by a single beam travelling both planes of incidence. That eliminates anisotropy in surface reflectance arising out of the difference in the efficiency of classical and conical diffraction.
The disclosed embodiments provide improvements over existing systems by providing an wafer inspection apparatus having an (r, xcex8) stage with a support surface on which a substrate coated with a patterned film may rest. A beam of light from a light source such as LED, or LD is directed toward the wafer/substrate surface such that it traverses beam paths in the plane of the paper as well as that in the orthogonal plane before being sensed by a detector. The detector that captures the effective reflection of the light source includes a receiver from which, a signal corresponding to the incident point on the substrate may be generated. The stage is capable of rotating and translating the substrate surface so that the focussed light spot can scan the entire wafer surface. Alternatively, the focussed light spot can be moved across the rotating wafer to scan its surface. A processor in communication with the detector is operable to generate image of the wafer surface based on reflectance data from a plurality of locations. Since the detector senses light from both (orthogonal) planes of incidence, the effective reflectivity of wafer surface with Manhattan geometry is homogenized and the resulting surface reflectance becomes more isotropic as shown in FIG. 2.
By using circularly polarized light or light with e-vector 45 degrees (45xc2x0) to the plane of incidence of 51, it is possible to propagate two beams counter to each other through the optical head. Surface image acquired with clockwise propagating beam should be identical to that acquired with counter clockwise propagating beam. However, defects such as irregularly shaped particles, debris or slurry aggregate, scratch with debris at the edge, or asymmetric pattern defects could scatter light differently for the symmetric illumination shown in FIG. 9. Consequently, the Grey level of the two defect images could be different. By subtracting the two images, background can be suppressed while enhancing the S/N of the defect image. See FIG. 10.
Exemplary embodiments of the present invention further comprise a wafer inspection apparatus having a stage with a support surface on which a wafer substrate may rest. The wafer stage is capable of moving the wafer in (x, y) or (r, xcex8) mode to achieve complete wafer scan. Polarized light from a monochromatic source is directed towards the wafer surface. The state of polarization of the beam entering PBS is either s- or p- or circular depending on the exemplary embodiment of the invention. Alternatively, both s- and p-polarization components are simultaneously present with the optical frequency of one of them shifted by xcex94f with respect to the other. The reflected light is sensed by detector(s). A processor in communication with the detector(s) can generate an image of the wafer surface based on reflectance data from a plurality of points generated via wafer (r, xcex8) scan. The polarizing beam splitters (PBS) along with the turning mirrors are configured such that every ray from the source that is directed toward PBS is propagated in two orthogonal planes of incidence. This helps to eliminate pattern dependent reflectance displayed by wafer (surface) with Manhattan geometry when scanned in (r, xcex8) mode. Consequently, uniformity of an acquired image is enhanced. In this embodiment, a defect in any orientation on a wafer surface should scatter light with same sensitivity when the wafer is scanned in (r, xcex8) or (x, y) mode. In accordance with an exemplary embodiment of the present invention, two images of the wafer surface taken simultaneously with two counter propagating beams. These images may be processed to enhance a defect signal while suppressing geometry generated background noise. Intensity non-uniformity due to interference pattern that exists above the wafer surface may be minimized. Normal illumination of the wafer surface may also be provided along with off-axis illumination. In addition, dark field inspection can also be implemented in any one of the described exemplary embodiments by locating off-axis detectors. Other embodiments describe phase-image inspection of wafer surface for detecting those defects that are insensitive to dark field or bright field inspection using two phase images that are 180xc2x0 apart in phase.